The present invention relates to a magnetic storage used for an auxiliary storage of a computer, or the like, a magnetic recording medium used for the magnetic storage, and a method of fabricating the magnetic recording medium.
With progress of information-oriented society, the amount of information daily used is steadily increasing. Demand for high density recording and large memory capacity for a magnetic storage is accordingly being stronger. An inductive head using voltage change in association with magnetic flux change with time is used as a conventional magnetic head. Both of recording and reproduction are performed by one head. In recent years, a composite head having a head for recording and a head for reproduction, in which an MR (magnetic resistive) head with higher sensitivity is used as the reproduction head, is rapidly increasingly used. In the MR head, change in electric resistance of a head device in association with change in magnetic flux leaked from a magnetic recording medium is used. A head with higher sensitivity using a very large magnetic resistive change (giant magnetic resistive effect or spin valve effect) which occurs in a plurality of magnetic layers laminated via non-magnetic layers is being developed. According to the head, change in electric resistance which is caused by change in relative directions of magnetization of the plurality of magnetic layers via the non-magnetic layers by the magnetic field leaked from a medium is used.
In magnetic recording media which are practically used at present, alloys containing Co as a main component, such as Co--Cr--Pt, Co--Cr--Ta, Co--Ni--Cr, and the like are used for a magnetic layer. Each of the Co alloys has a hexagonal close-packed (hcp) structure in which a c-axis direction is an easy axis of magnetization, so that a crystal orientation such that the c-axis of the Co alloy is the longitudinal direction, that is, (11.0) orientation is desirable as a longitudinal magnetic recording medium for reversing the magnetization in the magnetic layer and recording. The (11.0) orientation is, however, unstable, so that when the Co alloy is formed directly on a substrate, such an orientation is not generally obtained.
A method in which the fact that a Cr (100) plane having a body-centered cubic (bcc) structure has good lattice matching with a Co (11.0) plane is used, a (100) orientated Cr-undercoating layer is first formed on a substrate, and a Co alloy magnetic film is epitaxially grown, thereby obtaining the (11.0) orientation such that the c-axis of the Co alloy magnetic film is orientated to the in-plane direction. Also, a method in which a second element is added to Cr to improve the crystal lattice matching performance in the boundary face between the Co alloy magnetic film and the Cr undercoating layer and intervals of lattices in the Cr undercoating layer are widened is used. The Co (11.0) orientation is further improved and coercive force can be increased. There are examples of adding V, Ti, and the like.
Another factor necessary to realize high recording density is reduction in noises as well as increase in coercive force of the magnetic recording medium. Since the MR head has extremely high reproduction sensitivity, it is suitable for high density recording. However, the MR head is sensitive not only to reproduction signals from the magnetic recording medium but also to noises. Consequently, in the magnetic recording medium, it is requested to reduce noises more than a conventional technique. It is known that in order to reduce the medium noise, it is effective to fine and uniform the grain size of the magnetic film or the like.
As an importance request for the magnetic disk medium, improvement in shock resistance can be mentioned. Especially, a magnetic disk apparatus is mounted on a portable information device such as a notebook-sized personal computer or the like in recent years, so that improvement in the shock resistance is very important subject from the viewpoint of improving reliability. A glass substrate whose surface is strengthened or a crystallized glass substrate is used in place of a conventional Al alloy substrate to which Ni--P is plated on the surface, thereby enabling the shock resistance of the magnetic disk medium to be improved. Since the surface of the glass substrate is smoother than that of the conventional Ni--P plated Al alloy substrate, it is advantageous to reduce floating spacing between a magnetic head and the magnetic recording medium and is suitable to obtain high recording density. In case of using the glass substrate, however, problems of poor adhesion with the substrate, invasion of impurity ion from the substrate or absorption gas on the surface of the substrate into the Cr alloy undercoating layer, and the like occur. As a countermeasure, any of various metal films, alloy films, oxide films is formed between the glass substrate and the Cr alloy undercoating layer.
Japanese Patent Application Laid-Open Nos. 62-293511, 2-29923, 5-135343, and the like are techniques related to the above.
It is known that, as mentioned above, reducing and uniforming the grain size of the magnetic film is effective to reduce the medium noise. However, when a magnetic disk apparatus was produced experimentally by combining a magnetic recording medium with a recording density of about 900 megabits per square inch and a high-sensitive MR head according to the conventional technique, sufficient electromagnetic conversion characteristic by which 1 gigabit or higher recording density per square inch can be obtained could not be obtained. Especially, when the glass substrate was used as a substrate of the magnetic recording medium, poor electromagnetic conversion characteristic in a high recording density area was resulted. The cause was examined and it was found that the Cr alloy undercoating layer formed directly or via various metal or alloys as used in the conventional techniques on the glass substrate was not orientated as strong (100) as that in the case where it was formed on the Ni--P plated Al alloy substrate. A crystal plane except for (11.0) of the Co alloy magnetic film is grown in parallel to the substrate and the in-plane orientation of the c-axis as an easy axis of magnetization was small. Thus, the coercive force was reduced and a reproduction output with the high density recording deteriorated. In the case of using the glass substrate, the grain in the magnetic film was larger than that of the Al alloy substrate and the distribution of grains was larger by 20 to 30%. The medium noise was therefore increased and the electromagnetic conversion characteristic deteriorated. Even if an amorphous film or a fine crystal film disclosed in Japanese Patent Application Laid-Open No. 4-153910 was formed between the glass substrate and the undercoating layer, the size of the grain in the magnetic film was sometimes reduced to a certain degree but was not sufficiently reduced. It was not effective with respect to the reduction in the grain distribution, and preferable electromagnetic conversion characteristic could not be obtained.